Photochemical reactions dimerisations

Scheme 1.1. Some characteristic reactions of radicals. A and B are any two types of reactive radical. A = B reaction [1]+ is a dimerisation. The reverse [ 1 ] is bond homolysis and may be induced thermally or photochemically. Reaction [2] represents addition to an alkene derivative. Reaction [3], hydrogen atom transfer, is one of the most important displacement reactions of radicals.

The construction and properties of monolayers has been well documented by Kuhn (1979) and the photochemical reactions which occur in such systems reviewed (Whitten et al., 1977). Molecules in monolayers are usually ordered and in the case of rru/i -azastilbenes irradiation of the ordered array produces excimer emission and dimers (Whitten, 1979 Quina et al, 1976 Quina and Whitten, 1977). This contrasts with what is found when the fra/jj-isomers of such compounds are incorporated into micelles. In such systems the predominant reaction is cis-trans isomerisation excimer emission is lacking. It is suggested that the lack of isomerisation in the fatty acid monolayers is due to the tight packing and consequent high viscosity of such systems. Styrene also dimerises in a fatty acid monolayer. Interestingly, the products formed on photo-oxidation of protoporphyrins are dependent upon whether the reaction is carried out in a monolayer or a micelle (Whitten et al., 1978). Zinc octa-ethylporphyrin exhibits excimer emission in monolayers (Zachariasse and Whitten, 1973). Porphyrins are photoreduced by amines in monolayers (Mercer-Smith and Whitten, 1979). Electron-transfer reactions have been carried out with monolayers of stearic acid containing chlorophyll and electron acceptors such as quinones (Janzen et al., 1979 Janzen and Bolton, 1979). 

DNA absorbs UV light with a wavelength of <260 nm, leading to photochemical reactions, for instance, the dimerisation of pyrimidine (Eq. 9.2). 

For most of the compounds examined the excited state has an enhanced dipole moment compared with the ground state. This reflects the greater basicity of the double bond of the excited vinylarene and rationalises the fast and efficient protonation reaction which is observed. 9-Vinylanthracene was found to be an exception in that the excited state dipole moment was determined to be zero. This correlates with the fact that the dominant photochemical reaction of vinylanthracene is dimerisation to give (297) rather than hydration. 

Of a comparatively small range of photochemical reactions described for quinolines and isoquinolines, perhaps the most intriguing are some hetero-ring rearrangements of quaternary derivatives, which can be illustrated by the ring expansions of their A-oxides to 3,1-benzoxazepines. As with 2-pyridones, 2-quinolones undergo 2h-2 photo-dimerisation involving the C-3-C-4 double bond. ... 

Chakraborty (18) as well as by Joule (63). Recently de Mayo et al. (i) showed that vinyl carbazole (171) on photochemical reaction in CdS dispersed medium undergoes dimerisation leading to a cyclobutone (172) to which two carbazole units are attached. 

Most of a, P unsaturated compounds undergo photochemical change like migration of the alkenic jt-bond to give a p, y-unsaturated ketones, dimerisation by photochemically allowed [x s+tc s] cycloaddition, rearrangement and cis-trans isomerisation. These are among the commonest photochemical reactions of a, P-unsaturated ketones. 

This is a general situation for thermal, concerted additions those involving Ane + 2ne systems proceed readily, e.g. the Diels-Alder reaction, whereas those involving 2ne + 2ne systems, e.g. the cyclo-dimerisation of alkenes, do not. We might, however, expect that photochemical cyclodimerisation of alkenes would be symmetry... 

Metal complexes can occasionally give good results as catalysts in Diels-Alder reactions, as for butadiene and isoprene dimerisations with Fe(CO)2(NO)2 and butadiene addition to 2-butyne in the presence of Fe (cyclooctatetraenela . However, these catalysts usually promote formation of several products in non-specific manner, through a two-step mechan-ism - , not dissimilar to most photochemical processes (see following section). 

However, it may be anticipated that the portion of photochemical product, which structurally corresponds to the Diels-Alder adduct, does not do so stereochemically. In the photosensitised dimerisation of cyclopentadiene, CTo-dicyclopentadiene is obtained in about the same amount as the endo-isomer, together with an equal amount of tro/ji-1,2-cycloadduct , while the kinetic product of the thermal reaction is exclusively e/i(/o-dicyclopentadiene. 

The photochemical dimerisation of a, P-unsalurated ketones provides a further class of [ji +jt ] cycloaddition and is demonstrated for 3-methylcyclopent-2-enone and 3-methylcyclohex-2-enone, with the formation of head to head dimers and head to tail dimers. The a, (5-unsaturated enone absorbs light of wavelength 300 nm and get n,n excitation. The reaction is believed to occur from triplet state because ISC is efficient. While it,n triplet excited enone also cause dimerisation. 

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